1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser beam printer or a copying machine, and particularly, to a belt applicable to the apparatus.
2. Description of the Related Art
Color image forming apparatuses are now in practical use in which toner images formed on a plurality of photoconductive drums are superimposed. One type of color image forming apparatus is a tandem direct-transfer multicolor image forming apparatus that forms a multicolor image by directly transferring toner images onto a recording material conveyed by an electrostatic transportation belt (hereinafter, the belt will be abbreviated as an “ETB”, and the apparatus will be referred to as an “ETB system”). In the ETB system, a plurality of photoconductive drums are individually charged by charging means, and electrostatic latent images are respectively formed on the photoconductive drums by exposure means. Then, toner negatively charged by friction charging of developing means is applied onto the electrostatic latent images on the photoconductive drums to form toner images, and the toner images are directly transferred onto a recording material transported by the ETB. The recording material having the toner images is delivered from the ETB to a fixing device, and the toner images are fixed on the recording material. Through this procedure, a full-color toner image is formed on the recording material.
Another type of color image forming apparatus is an intermediate-transfer multicolor image forming apparatus that forms a multicolor image by primarily transferring toner from photoconductive drums onto an intermediate transfer belt (hereinafter abbreviated as an “ITB”, as required), and then secondarily transferring the toner onto a recording material (hereinafter, the apparatus will be referred to as an “ITB system”). In the ITB system, the ITB is in contact with the photoconductive drums at primary transfer positions. Toner images formed on the photoconductive drums are transferred onto the ITB (primary transfer), and are then transferred from the ITB onto a recording material at a secondary transfer position (secondary transfer). The ETB and the ITB are sometimes generically and simply referred to as a transfer belt or belt.
In these image forming apparatuses, the density and position of toner on the transfer belt are detected in order to achieve high-color reproducibility and high-definition images. Detection of the density and position is typically performed with an optical sensor because of its low cost and high accuracy. A toner patch is formed on the transfer belt, and the presence and density of toner is detected by using the toner patch, that is, on the basis of the difference in reflectance between a toner portion and a portion having no toner. The position and density of the toner are adjusted according to a detection result.
A reflective optical sensor is often used as the optical sensor. In the reflective optical sensor, the intensity of light, which is incident on the transfer belt at a fixed angle (e.g., 30°) and is regularly reflected by the transfer belt, is monitored by a detector such as a phototransistor. The light is emitted from an inexpensive and long-life light emitting diode serving as a light source, and has a wavelength within a range over the visible region and the near-infrared region, that is, within a range of 400 to 1000 nm.
The transfer belt is required to have various characteristics, for example, reflectance needed to detect the density and position of toner, wear resistance and excoriation resistance needed to prevent the surface of the belt from being worn or roughened by friction with toner, carrier, a cleaning blade, and a recording material, and a sliding characteristic for preventing stick slip with respect to cleaning blades and the photoconductive drums.
In order to ensure these characteristics, it is known to form a multilayer structure by coating the surface of the transfer belt. This method realizes a high-performance inexpensive transfer belt.
However, depending on the layer structure, there is a need to prevent an adverse effect of light interference between a plurality of layers. For example, when the density and displacement of toner provided on a multilayer belt or image bearing member are detected by an optical sensor, interference reduces detection accuracy.